In recent years, the technical improvement of image display devices has made it possible to display precise CG (computer graphics) images and fully realistic high-definition images of nature. However, there has been a daily growing demand that higher gradation and higher definition images should be displayed.
Further, since image display devices have been applied to every field, e.g., to on-board units and medical devices as well as household appliances, there has been a very high demand for qualities including reliability. A liquid crystal panel serving as a liquid crystal display device has also been increasingly required to display higher definition images, so that a liquid crystal driver LSI to be provided in the liquid crystal panel has been made to have more outputs and to provide more gradations.
In order to carry out this gradation display, a liquid crystal driver has outputs each containing a DA converter and each designed to output a gradation voltage. This operation is a little more fully explained below. First, see FIG. 12, which shows a structure of a conventional liquid crystal driver.
The liquid crystal driver shown in FIG. 12 sequentially samples input data (6 bits or more/1 output) respectively corresponding to liquid crystalline system outputs, loads and latches data corresponding to the number of outputs, and input the data to DA converters via a level shifter. Since this liquid crystal driver is a well-known structure, its operation is not fully explained.
Each of the DA converters selects a gradation level for each output, and outputs, via an operational amplifier provided for each output, a gradation level generated by a reference voltage generation circuit (ladder resistor). FIG. 13 shows the ladder resistor that is to be used as the reference voltage generation circuit. Generally, a desired gradation level is generated for each gradation by dividing the ladder resistor into resistors.
As for the aforementioned input data, a 6-bit DAC, an 8-bit DAC, and a 10-bit DAC make it possible to display 64 gradations, 256 gradations, and 1024 gradations, respectively.
As an LSI for use as a liquid crystal driver has been made to provide more gradations, high-precision measurement has become indispensable when the liquid crystal driver is tested for the securement of its quality. That is, a more highly precise test needs to be conducted to see whether gradation voltages respectively outputted from DA converters are all correct and whether the gradation voltages are equal to one another.
If devices under test DUT have the same power supply voltage, then measurement accuracy needs to be quadrupled in cases where the performance of output terminals has been improved from 64 gradations to 256 gradations.
The following explains an example of a method for testing an LSI for use as a liquid crystal driver as a device under test DUT. Assume that the device under test DUT is an LSI for use as a liquid crystal driver having m output terminals each containing an n-gradation DA converter for selecting and outputting one of n voltage levels.
FIG. 14 is a diagram schematically showing a gradation testing method (system configuration) using a high-precision voltmeter. This system is constituted by a device under test DUT and a semiconductor testing device (tester).
The semiconductor testing device inputs a predetermined input signal to the device under test DUT, and determines whether an output signal from the DUT is good or bad. In the foregoing system configuration, a first-gradation level is outputted by supplying a predetermined input signal to the device under test DUT (i.e., the liquid crystal driver) with the use of the semiconductor testing device (tester).
Thereafter, the high-precision analog voltmeter is used to sequentially measure a first-gradation voltage for each output until the mth output, and the measurement results are sequentially stored in a memory contained in the semiconductor testing device (tester).
This operation is repeated the number of times corresponding to the number of gradations n. Finally, data corresponding to all the outputs and all the gradations are stored in the memory. As a result, data corresponding to the number obtained multiplying the number of gradations n by the number of outputs m are stored.
The data stored in the memory are subjected to a predetermined calculation with the use of a calculation device contained in the semiconductor testing device (tester), and a test is conducted on the shift length by which gradation voltages of each output terminal are shifted from one another and on the variations (uniformity) in gradation voltage among the output terminals.
In such a liquid crystal driver test, an increase in the number of gradations makes it necessary to more highly precisely measure gradation voltages.
As evidenced by the above explanation, basic test items to be conducted in testing a liquid crystal driver according to the foregoing method include a test item as to whether a voltage of each output terminal falls within a desired range for each gradation and a test item as to whether a variation between terminals falls within a desired range.
Furthermore, supplementary test items to be conducted in addition to the basic test items include: a functional test on basic operation; a test on AC characteristics such as an operation margin, consumption current, and delay time; a test on minute leak current; and other tests.
These tests are designed to detect a defect in a liquid crystal driver. In addition, it is necessary to improve screening accuracy by revealing a potential defect factor. As described above, liquid crystal drivers are applied to products such as on-board units and medical devices as well as household appliances, and there has been a very difficult request for qualities including reliability.
In order to respond to a request for the higher quality of devices in addition to the improved function of devices, it has become necessary to conduct a burn-in test for revelation of a potential defect. At present, a burn-in test on liquid crystal drivers is conducted with the liquid crystal drivers taking the form of a package or a wafer.
The following fully explains a case where a burn-in test is conducted on liquid crystal drivers taking the form of a wafer.
As described above, a liquid crystal driver sequentially samples input data (6 bits or more/1 output) respectively corresponding to liquid crystalline system outputs, loads and latches data corresponding to the number of outputs, and input the data to DA converters via a level shifter. Each of the DA converters selects a gradation level for each output, and outputs, via an operational amplifier provided for each output, a gradation level generated by a reference voltage generation circuit (ladder resistor).
In a burn-in test, a circuit is entirely activated by carrying out the foregoing operation at overload (under a desired voltage condition and in a hot environment, etc.). By conducting such a burn-in test for a predetermined period of time, it is possible to reveal a potential defect factor in each chip.
In a burn-in test, output terminals for a status monitor are needed in addition to input data for setting gradation levels, a power supply, and a ground. For example, in case of an 8-bit (256-gradation) driver, a total of 52 signal supply inputs are needed.
After the burn-in test has been conducted, the aforementioned test for detection of a defect is conducted. By conducting the tests in such a flow, it is possible to respond to a request for the high quality of devices.
The duration of the test for detection of a defect can be reduced in various special ways. However, how long devices under test are activated is the key to a burn-in test for revealing a potential defect by activating the interior of the devices under test. Even in various special ways, it is difficult to reduce the time during which each of the devices under test is activated, and such a time reduction causes a decrease in test capacity and an increase in test cost.
Here, an effective way to reduce the time and cost required for a burn-in test is to conduct a simultaneous burn-in test on a large number of devices (such a simultaneous burn-in test being hereinafter referred to as “multiple simultaneous test”). However, as described above, a recent liquid crystal driver made to have more outputs and to provide more gradations has a large number of input terminals and output terminals (especially input terminals). This makes it difficult to conduct a multiple simultaneous test.
That is, a semiconductor device, typified by a liquid crystal driver, which includes a large number of input terminals, output terminals, and power supply terminals has a large number of terminals. Therefore, in testing a large number of such semiconductor devices simultaneously, test signal terminals (hereinafter referred to as “pin electronics (PEs)”) of a semiconductor testing device are used. For this reason, the number of PEs of a testing device restricts the number of semiconductor devices to be subjected to a multiple test.
Further, in testing a semiconductor device fabricated on a wafer, a wafer probe card (WPC) is used via which a PE of a semiconductor testing device and an electrode terminal of a semiconductor device to be tested are electrically connected to each other. However, a connection of a large number of probe needles onto the electrode terminal causes a shortage of physical space in which the needles are mounted, thereby making it difficult to realize a WPC that makes it possible to conduct a multiple test. For this reason, the more terminals a semiconductor device has, the more difficult it is to conduct a multiple simultaneous test. This is explained below with reference to FIGS. 15 and 16.
FIGS. 15(a) and 15(b) show a structure of a probe card 110 being used for conducting a normal function single test on a conventional liquid crystal driver 100. In FIGS. 15(a) and 15(b), the probe card 110 and the liquid crystal driver 100 are in contact with each other. FIG. 15(a) is a top view of the probe card 110, and FIG. 15(b) is a side view of the probe card 110.
The probe card 110 includes input-terminal-side probe needles 111, output-terminal-side probe needles 112, a probe card substrate 113, and probe-fixing pedestals 114. In a test on the liquid crystal driver 100, the input-terminal-side probe needles 111 for controlling the liquid crystal driver 100 and the output-terminal-side probe needles 112 for voltage-driving the liquid crystal are electrically connected to an external tester, and the liquid crystal driver 100 is subjected to an operation test.
All the signals necessary for causing the liquid crystal driver 100 to operate are inputted to the liquid crystal driver 100 from an outside source; therefore, a probe card to be used for the liquid crystal driver 100 needs to have probe needles respectively corresponding to all the input terminals of the liquid crystal driver 100. If the liquid crystal driver 100 is identical in structure to a driver section 10 shown in FIG. 1, then a total of 52 input-terminal-side probe needles 111 respectively corresponding to all the input terminals (namely a CK terminal, an SP terminal, 48 DATA terminals, a REV terminal, and an LS terminal) are needed. These input-terminal-side probe needles 111 are provided so as to correspond to a longer side (left in the figure) of the liquid crystal driver 100.
Further, the output-terminal-side probe needles 112 are similarly provided so as to correspond to a longer side (right in the figure) of the liquid crystal driver 100, which longer side is opposite the longer side to which the input-terminal-side probe needles 111 are provided so as to correspond. Further, a mainstream type of current liquid crystal driver includes a total of 384 to 720 output terminals, and the optimum number of output terminals is selected depending on various uses of panels.
The probe needles 111 and 112 are fixed to the probe card substrate 113 via the probe-fixing pedestals 114, and tips of the probe needles 111 and 112 and input pads of the liquid crystal driver 100 are brought into contact with each other so as to be electrically connected to each other, respectively. Thus, a test is conducted on the input terminals and output terminals of the liquid crystal driver 100 by bringing all the device-specified input and output terminals into contact respectively with the probe needles 111 and 112 of the probe card 110.
As shown by the foregoing condition of contact, in conducting a multiple simultaneous test, the number of probe needles needs to correspond to the number obtained by the number of all probe needles per liquid crystal driver by the number of DUTs (devices under test). Under conditions where DUTs are brought into contact for a normal test, a multiple simultaneous test becomes difficult due to restrictions imposed by the problem of physical space in which the probe needles are mounted and the method for fixing the probe needles.
In cases where a test on the liquid crystal driver 100 is a burn-in test, it is possible, as shown in FIG. 16, that only the input-terminal-side probe needles 111 are brought into contact with the liquid crystal driver 100. That is, the purpose of a burn-in operation is to serve as a commonly-known technique to put operation stress on a DUT, and to achieve early revelation of a progressive defect mode. Therefore, in a burn-in test, it is only necessary to control operation of the liquid crystal driver 100 serving as a DUT, so that it is only necessary to set the input-terminal-side probe needles 111. However, as described above, even with a probe card 110 having 52 input-terminal-side probe needles 111, it is difficult to conduct a multiple simultaneous burn-in test.
Japanese Unexamined Patent Application No. 218936/2004 (Tokukaihei 4-218936; published on Aug. 10, 1992; hereinafter referred to as “Patent Document 1”) discloses a semiconductor device for use as a liquid crystal driver, into which semiconductor device a burn-in control circuit is incorporated in order to reduce the number of input terminals for use in burn-in operation control. FIG. 17 is a diagram showing a semiconductor device, disclosed in Patent Document 1, which is used as a liquid crystal driver.
The liquid crystal driver of Patent Document 1 is arranged as follows. That is, the liquid crystal driver is set to a test mode when a test signal is inputted to the NTEST terminal. Then, the CR oscillation circuit 120 causes self-oscillations of the liquid crystal driver. In accordance with the self-oscillation clock, the burn-in control circuit 130 generates a test signal. This makes it possible to conduct a burn-in test without supplying a test control signal to a large number of logic input terminals from an outside source.
However, the internal state of the liquid crystal driver of Patent Document 1 is set in accordance with the clock signal generated due to the self-oscillations, the liquid crystal driver cannot be set to a given state at a given timing. Further, the frequency is fixed. This causes such a problem that an IDDQ test for securement of high quality cannot be conducted in a given state. The “given state” refers, for example, to a case where a memory cell is set to 1 or 0, and high quality can be secured by conducting an IDDQ test in each state. Further, high quality can be further secured by conducting an IDDQ test with an adjacent bit set as an inversion bit.
Furthermore, the liquid crystal driver of Patent Document 1 is designed to simplify a device structure needed for a burn-in test, and is not designed for a multiple simultaneous test on a large number of devices under test. As a matter of fact, it is impossible to conduct a multiple simultaneous test on a large number of such liquid display drivers. The reason for this is as follows.
See a case where an attempt is made to conduct a multiple simultaneous test on a large number of liquid crystal drivers of Patent Document 1. In this case, even when input signals to the drivers are synchronized, subsequent operation in each driver is based on a clock signal generated due to self-oscillations of the driver. Therefore, output signals from the drivers cannot be synchronized. In a multiple simultaneous test, output signals from the drivers need to be synchronized. Therefore, it is impossible to conduct a multiple simultaneous test on a large number of liquid crystal drivers of Patent Document 1.
Thus, semiconductor devices such as liquid crystal drivers have been applied to fields, such as automobiles and medical devices as well as conventional game machines and portable electronic devices, in which high reliability is required. For securement of high quality, it has become necessary to introduce a burn-in test that requires a long inspection time. However, as described above, the conventional technique cannot make it possible to conduct a multiple simultaneous burn-in test. This causes an increase in time required for an inspection process. This makes it difficult to respond to customers' demands in terms of shipping date, high quality, and product price.